Over Voltage Ground Relay Activation 1

[charz]

The phase to phase voltages are consistent with each other. The phase to neutral voltages are not constant.
The system is ungrouded system.There is no ground fault but the numerical ground over voltage relay gets activated.

The Potential transformer connected is of star-star (grounded at both sides). The Phase voltage measured at
the secondary of the potential transformer is constantly fluctuating but the phase to phase voltages are normal. Is it ferro resonance effect?

 

[davidbeach]

I'm pretty sure that your phase-neutral voltages aren't wandering around; and you wouldn't know since you're looking at phase-ground voltages. The neutral-ground voltage is wandering around and has no real meaning. Very minor imbalances can move the neutral around relative to ground without meaning anything. All you have to "hold" the neutral-ground voltage in place is the parasitic capacitance between the phases and ground; switch a bit of the system in or out and you can change those capacitances. Mostly just ignore the phase-ground voltages as long as none of the phases are solidly at zero volts to ground.

 

[charz]

Thanks David,

"Mostly just ignore the phase-ground voltages as long as none of the phases are solidly at zero volts to ground"

I cannot ignore this as it is operating the 59N element in the protection relay.

 

[davidbeach]

It could just be too sensitive. The system isn't "ungrounded", rather it is grounded through the parasitic capacitances that you can't control. Expecting tight regulation of an unstable value isn't realistic, but it gets tried every now and then. What magnitude of V1 would the relay see if everything was perfectly balanced and what value of 3V0 does it trip at?